This invention relates generally to stereo television receivers and particularly to circuits for muting the stereo television receiver sound system to prevent noises, such as pops and clicks, due to changes in tuning or monaural/stereo audio transitions from reaching the viewer.
As is well known, stereo television receivers operate with a suppressed carrier audio signal and a pilot subcarrier signal for recovery of the suppressed carrier. Should the transmitted pilot signal be removed for monaural transmissions, or be dropped for any reason, the television receiver switches from the stereo operating mode to the monaural operating mode, which often results in an audio click or pop in the receiver speakers. Also when tuning among the various television signals, the receiver sound channels are muted to avoid annoying noises due to the absence of a television signal.
Conventionally, the audio system can be muted by connecting the emitter-collector path (output circuit) of a transistor between the audio signal channel and a source of reference potential, such as ground. The base (or input circuit) of the transistor is coupled to a suitable switching signal and the transistor is turned on (driven conductive) when audio muting is desired. Most modern television receivers include a microprocessor for providing various logic level control signals for tuning, video processing, and other miscellaneous control functions. These logic level control signals vary between a logic high first voltage level and a logic low second voltage level. The high level must be sufficient to drive the transistor into saturation to suppress the audio signal. A difficulty arises, however, since the logic low signal level in a transistor-to-transistor logic (TTL) system may be as high as 0.8 volts DC, which will turn the transistor partially on and cause attentuation and distortion of the normal audio signal. The prior art connected a diode in series with the transistor base electrode. The diode exhibits leakage and the voltage at the base of the transistor can still range from 0.3 to 0.6 volts DC. If the audio signal goes sufficiently negative, the transistor will clip the audio and produce distortion.
The present invention utilizes a light emitting diode (LED) which is capable of handling a larger audio signal and which exhibits no leakage current in its off state. Thus it maintains the transistor base at 0 volts DC which allows the audio signal to go to -0.7 volts DC before clipping (distortion) occurs. The circuit of the invention also uses two separate mute circuits, of different delay characteristics, for monaural/stereo switching and for tuning. The "tuned signal" delay circuit has approximately a 700 millisecond delay and is responsive to the absence of horizontal sync for muting the audio system. The monaural/stereo delay circuit has approximately a 150 millisecond delay and is responsive to switching of the LED indicator light for muting the audio. The long delay for tuning is sufficient to enable the receiving circuitry to determine whether a stereo audio signal is being received or not. The short delay for monaural/stereo transitions guards against dropouts of the pilot subcarrier which sometimes occur. The combination of the two delay circuits produces an attractive audio muting circuit arrangement for a stereo TV receiver utilizing TTL logic level signals.